Surface wave signal storage device

ABSTRACT

Surface acoustic waves are propagated along piezoelectric material, in response to an electrical signal applied to an input transducer. During propagation along the material, the propagation surface of the piezoelectric material is flooded with a short burst of electrons, causing the emission of low energy secondary electrons. These electrons are immediately attracted from the negative to the more positive adjacent regions of the surface, neutralizing any potential variations on the surface. After the acoustic waves thereon have subsided, electrons trapped on the surface are left behind in a frozen pattern corresponding to the pattern of the acoustic waves. The acoustic waves can be reformed by again flooding the surface with a short burst of electrons. Output transducer means act to transform the reformed waves into an electrical signal.

United States Patent Kazan 51 May 9, 1972 [54] SURFACE WAVE SIGNAL STORAGE DEVICE [72] Inventor:

[73] Assignee:

Benjamin Kazan, Briarcliff, N.Y.

International Business Machines Corporation, Armonk, N.Y.

June 3, 1970 References Cited U.S.Cl. ..340/l73 R,310/8.l, 313/89, 313/313, 315/83, 315/55, 333/30 R Int. Cl. "Gllc 27/00, G1 1c 13/00 Field of Search ..340/1 73 R, 173 MS; 333/29 R,

333/29 S, 30 R, 72, 99 R; 315/85, 55; 313/89, 313;

Crabbe ..333/72 UX Okamura ..333/30 R Primary E.\'aminerEugene G. Botz Assistant Examiner-R. Stephen Dildine, Jr. Attorney-Hanifin and Jancin and George Baron [5 7] ABSTRACT Surface acoustic waves are propagated along piezoelectric material, in response to an electrical signal applied to an input transducer. During propagation along the material, the propagation surface of the piezoelectric material is flooded with a short burst of electrons, causing the emission of low energy secondary electrons. These electrons are immediately attracted from the negative to the more positive adjacent regions of the surface, neutralizing any potential variations on the surface. After the acoustic waves thereon have subsided, electrons trapped on the surface are left behind in a frozen pattern corresponding to the pattern of the acoustic waves. The acoustic waves can be reformed by again flooding the surface with a short burst of electrons. Output transducer means act to transform the reformed waves into an electrical signal.

9 Claims, 2 Drawing Figures l2 n l l f i l ii i v i l i Lff j 55 1? 17 PIITENTEDIIIY 9 I972 3,662.355

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I8) I I6/ I /07 07 I/ I II" II" 2 I0- I QQO F G. 2 m 28 ACOUSTIC I x I, STORAGE DEVICE I 54 24 u so I PUT b ACOUSTIC OUTPUT N OM STORAGE coMPARIs0N ma 26 DEVICE 2 CIRCUIT WWW. 32 ACOUSTIC STORAGE Jmo DEVICE N INVENTOR BENJAMIN KAZAN ATTORNEY SURFACE WAVE SIGNAL STORAGE DEVICE BACKGROUND OF THE INVENTION Structures which delay a signal by the time taken for an acoustic wave to pass between two transducers in a solid medium have found various uses in technology. Such acoustic wave delay devices are attractive since the velocity of propagation is many orders of magnitude less than the velocity of propagation of electromagnetic waves in a vacuum or electrical signals along a transmission line. A class of devices that has beenfound particularly for delay lines is based on the propagation of surface acoustic waves. A surface wave device is one in which the acoustic wave motion is confined to a thin layer very close to the surface. One favorable feature of surface waves is that they can be guided along the surface in accordance with variations in dielectric constants of the material, and a considerable art has grown up around such guiding means. See, for example, U. S. Pat No. 3,406,358 of H. Seidel et al. and U. S. Pat. No. 3,200,354 ofD. L. White.

The present invention, like prior art devices, employs a piezoelectric material, such as, but not restricted to, quartz on whose surface is launched a train of surface waves.- Input signals are applied to a transducer provided at one end of the quartz slab and are absorbed at the opposite end of the quartz slab by a second,suitably-matched, output transducer element. Since the quartz is piezoelectric, an electric field or pattern of potentials is propagated along the surface of the quartz, which field or pattern corresponds to the elastic wave (surface acoustic wave) propagated along the surface.

At the time that the launched surface acoustic wave reaches the output transducer, the surface of quartz that is carrying the launched wave is flooded with an intense, short burst of electrons, whose duration is, for example, a few nanoseconds, the latter causing the emission of low energy secondary electrons. These electrons are immediately attracted from the negative to the more positive adjacent regions of the surface, neutralizing any potential variations on the surface of the quartz.

After the acoustic surface wave has subsided, electrons trapped on the surface are left behind in a pattern corresponding to the magnitude and spacing of the individual surface waves of the propagating signal. This'pattern can be retained for long periods, for example, days, depending upon the insu- Iating quality of the piezoelectric material. Information can thus be frozen in and does not depend upon the continuous recirculation of an acoustic wave for storing data.

When desired, stored information can be retrieved by again subjecting the quartz surface to a flood of electrons for a short period, for example, of the order of nanoseconds. The secondary electrons produced at the surface will then again be attracted from the negative to the more positive areas charging the entire surface to a uniform potential. This charging step causes a sudden release of the stresses on the surface of the quartz which had been produced by the trapped charge and causes a surface wave to initiated which is identical to the original acoustic wave that existed before storage. The output transducer on the quartz surface will then sense a time-varying signal corresponding to the initial input signal which had been launched and previously stored.

Whereas known prior art devices can dynamically store an acoustic wave by recirculating the wave through the delay line, they do not provide any static storage, nor do they allow an arbitrary amount of delay to be obtained. Thus, the device to be described in greater detail hereinbelow can be used as a memory buffer or storage device having an arbitrary delay time.

Thus, it is an object to provide a novel storage and readout technique for a surface acoustic wave.

Yet another object is to record information carried by a surface acoustic wave using the interaction of an electron beam and piezoelectricity of the carrier of said acoustic wave.

A further object is to provide means for processing data when such data is in the form of an acoustic surface wave.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic showing of a preferred embodiment of the memory device for storing information contained in a surface acoustic wave.

FIG. 2 is a schematic circuit for comparing the similarities or differences between stored acoustic waves.

In FIG. 1 is shown a piezoelectric rod 2 or sheet, such as quartz, having an input transducer 4 and a matched output transducer 6. These are, for example, commonly used interdigital transducers. Electrical signals applied to the input terminals of leads 8 and 8' will be converted to mechanical stresses by transducer 4 which are propagated along the surface of rod 2 as acoustic waves, the latter being converted by transducer 6 into electrical signals that appear on leads l0 and 10' connected to a suitable sensing device.

Facing the upper surface of rod or sheet 2 of quartz is an elongated cathode 12, which may be heated to supply a source of electrons, that is parallel to that upper surface. The cathode 12 is capable of being rapidly set to one of two potentials by means of an electronic switch 14, illustrated as a mechanical switch for simplicity, either to a relatively low positive potential of +10 volts from battery 16 or to arelatively high negative potential of -l,000 volts by means of battery 18. Interposed between the top surface of the quartz acoustical conductor 2 and heated cathode12 is a grounded control grid 20 which may be in the form of a fine mesh or an electrode having a narrow slit running parallel to the cathode 12. When cathode 12 is set to 10 volts positive with respect to grounded control grid 20, no electrons reach the surface of the quartz 2; however, when cathode 12 is set to a potential of l ,000 volts, electrons pass through control grid 20 flooding the surface of the quartz rod uniformly with electrons.

To erase any previous information stored on the piezoelectric member 2, cathode 12 is momentarily switched to -l000 volts, flooding the quartz along the length with electrons. The secondary electrons produced then charge the surface of the quarts 2 uniformly to ground potential. After this, the cathode 12 is switched to +10 volts, via switch 14, cutting off electronic flow to quartz 2, and a surface wave is launched by applying electric signal voltages to leads 8-8. At any time during the propagation of the acoustic wave along the surface of the piezoelectric, the wave can be frozen in or stored by shifting switch 14 to the -1,000 V source for a fraction of a microsecond, flooding the piezoelectric surface with electrons. Secondary electrons created at the surface will then act to instaneously shift all surface elements of the quartz 2 to approximately ground potential, effectively cancelling the instantaneous local potentials on the piezoelectric surface which were produced by the acoustic surface wave.

After the surface wave has subsided, trapped electrons are left behind in a pattern corresponding to the positive crests of the surface waves. Since quartz is insulating, as well as piezoelectric, the resulting charge pattern is retained, providing storage, or memory, for long periods. The trapped electrons will produce in turn local electric fields and corresponding stresses on the surface of piezoelectric material 2 in accordance with the dynamic surface wave at the instant it was stored or frozen in. In other words, the information content of the acoustic wave is transformed into a static potential pattern on the quartz surface.

In order to read out such a static potential pattern at a later time, switch 14 is made to contact the negative terminal of battery 18 for a few nanoseconds or less to allow a flood of electrons to pass through grid 20 onto the quartz surface 2. The secondary electrons produced in the quartz 2 surface will then be again preferentially attracted to the more positive areas producing a uniform potential over the quartz surface.

This sudden discharging of local areas of the surface by flooding with electrons causes a sudden release of the stresses on the surface; as a result, a pattern of transient voltages is produced which causes a surface wave to be initiated whose information content is identical to that of the original acoustic wave that was initially launched. The transducer 6 at the end of the quartz 2 acoustic wave carrier will then sense a timevarying signal corresponding to the original input signal, but delayed in time at the will of the operator. Such time-varying signals can be displayed on an oscilloscope 22 or other suitable display well known in the act.

To accomplish proper recording and readout of the stored acoustic wave, the electron beam must be pulsed on for a period which is a small fraction of the period of the surface acoustic wave. If the electron pulse is longer, the information in the acoustic wave may be degraded and the output signal reduced.

Assume an acoustic surface wave having a frequency of 100 MHZ (period of 10 seconds) is to be captured as a static potential pattern on a piezoelectric surface and subsequently read out. Such should take place in a time that is, for example, one-tenth the period of that wave. Thus the electron pulse from cathode 12 must have a duration of 10 seconds or less. The current density required in the electron beam that floods the piezoelectric surface is determined by the capacitance of the surface layer of material in which the acoustic wave is propagating. Assuming that the propagation velocity of such a wave is 3X10 cm/sec, the wavelength will be 3X10 cm. For acoustic surface wave propagation, the effective depth of the wave in the transporting medium is roughly equal to its wavelength, or about 3 10'- cm.

The capacitance C in Farads is represented by the relationship where K is a dielectric constant, A is the area in sq. cm. of the surface being bombarded with electrons, d is the depth in cm. of the acoustic waves. Assuming K=4 and d= 3X10 cm, the capacitance has a value of lF/cm The charge, Q, required to discharge the surface is, Q CV= 1!, where V is the surface potential, I is the primary current density (assumed to be equal to the secondary current density) and t is the duration of the current pulse. Assuming a surface potential of 1 volt, a charging time of 10 sec. and a capacitance C lO' F/cm, a current density 1, is obtained of 0.1 amps/cm? Such current densities are readily available with state of the art electron emitters, for example, heated .oxide cathodes.

The unit of FIG. 1 can be employed wherever delays or memories are to be used employing surface acoustic waves. FIG. 2 is a schematic representation of how the invention of FIG. 1 can be used for signal comparison. Input switch 24 is an electronic switch that can rapidly switch an input signal to contacts a, b and n. An electric signal, carrying information, such as a TV signal, appears on input line 26 and is converted into an acoustic surface wave and stored in Acoustic Storage device No. 1 if switch 24 makes contact with contact a. In a similar manner, separate trains of input signals can be stored in the remaining Acoustic Storage Devices 2 n. The signals thus stored as frozen acoustic surface waves can be retained almost indefinitely. If desired, the switches 28, 30 and 32 can be maintained closed and a readout signal simultaneously produced from all the devices, No. 1, No. 2, No. n. These signals can then be sent to a comparison circuit 34 which adds or subtracts the various signals from each other. For example, successive TV input signals appearing on contacts a, b n could be first stored and then simultaneously compared in the circuit 34, whose output indicates any changes or differences between the input signals.

In prior acoustic delay devices, the delay time is determined by the length of the path traversed by the wave. In the new scheme described here an arbitrary amount of delay time can be introduced into the signal without changing the path length. Additionally, since storage of the surface acoustic wave is almost permanent or exceedingly long, storage times LII can be obtained without requiring dynamic recirculation of the stored information.

What is claimed is:

1. A storage device comprising a material having a piezoelectric surface,

means for launching an acoustic wave along said surface so as to produce electric fields on said surface corresponding to localized variations of said wave,

means for flooding said surface with electrons during the presence of said acoustic wave so as to produce a static potential pattern along said surface corresponding to said launched wave, and

output means for removing said static potential pattern.

2. The storage device in claim 1 wherein said piezoelectric surface is also insulating.

3. A storage device comprising a material having a piezoelectric surface, I

means for launching a surface acoustic wave having a given period along said surface so as to produce electric fields on said surface corresponding to localized variations of said wave, means for flooding said surface with electrons during the presence of said acoustic wave so as to produce a static potential pattern along said surface corresponding to said launched wave, said electron flooding taking place for a period that is a fraction of the period of said acoustic wave, and means for sensing an output acoustic wave corresponding to said static potential pattern, said output acoustic wave being a reconstitution of the initially launched surface acoustic wave and producible by flooding said surface with electrons. 4. The storage device of claim 3 wherein said electron flooding period is one-tenth or less than the period of said acoustic wave.

5. The storage device of claim 3 wherein said piezoelectric surface is quartz.

6. A storage device comprising a material having a piezoelectric surface,

input transducer means arranged to transform an electrical input signal into an acoustic wave which propagates along said surface so as to produce electric fields on said surface corresponding to localized variations of said wave,

means for momentarily flooding said surface with electrons during the presence of said acoustic wave so as to produce a static potential pattern along said surface cor responding to said launched wave, and

output transducer means arranged to transform a reformed acoustic wave corresponding to said pattern into an electrical output signal, said reformed acoustic wave being produced by flooding said surface with electrons.

7. A storage device comprising a material having a piezoelectric surface,

means for launching an acoustic wave on said surface and means for detecting said launched wave,

a cathode for momentarily flooding said surface with electrons,

a grid interposed between said cathode and said piezoelectric surface, and

means for supplying either a negative or a positive potential to said cathode with respect to said grid.

8. A device comprising a material having an insulating piezoelectric surface which surface has been set at a uniform potential,

means for launching an acoustic wave on said surface,

means for momentarily flooding said surface with electrons during the presence of said acoustic wave so as to produce a static potential pattern along said surface corresponding to said launched wave, and

output means to sense an output acoustic wave corresponding to said static potential pattern along said surface, said output acoustic wave being a reconsitution of the initial acoustic wave and producible by flooding with electrons said surface with static potential pattern.

9. In a device containing a piezoelectric insulating surface on which a charge pattern has been established,

means for flooding said surface with electrons to generate an acoustic surface wave corresponding to the charge pattern, and 5 means for sensing time-varying signals produced by said acoustic surface wave. 

1. A storage device comprising a material having a piezoelectric surface, means for launching an acoustic wave along said surface so as to produce electric fields on said surface corresponding to localized variations of said wave, means for flooding said surface with electrons during the presence of said acoustic wave so as to produce a static potential pattern along said surface corresponding to said launched wave, and output means for removing said static potential pattern.
 2. The storage device in claim 1 wherein said piezoelectric surface is also insulating.
 3. A storage device comprising a material having a piezoelectric surface, means for launching a surface acoustic wave having a given period along said surface so as to produce electric fields on said surface corresponding to localized variations of said wave, means for flooding said surface with electrons during the presence of said acoustic wave so as to produce a static potential pattern along said surface corresponding to said launched wave, said electron flooding taking place for a period that is a fraction of the period of said acoustic wave, and means for sensing an output acoustic wave corresponding to said static potential pattern, said output acoustic wave being a reconstitution of the initially launched surface acoustic wave and producible by flooding said surface with electrons.
 4. The storage device of claim 3 wherein said electron flooding period is one-tenth or less than the period of said acoustic wave.
 5. The storage device of claim 3 wherein said piezoelectric surface is quartz.
 6. A storage device comprising a material having a piezoelectric surface, input transducer means arranged to transform an electrical input signal into an acoustic wave which propagates along said surface so as to produce electric fields on said surface corresponding to localized variations of said wave, means for momentarily flooding said surface with electrons during the presence of said acoustic wave so as to produce a static potential pattern along said surface corresponding to said launched wave, and output transducer means arranged to transform a reformed acoustic wave corresponding to said pattern into an electrical output signal, said reformed acoustic wave being produced by flooding said surface with electrons.
 7. A storage device comprising a material having a piezoelectric surface, means for launching an acoustic wave on said surface and means for detecting said launched wave, a cathode for momentarily flooding said surface with electrons, a grid interposed between said cathode and said piezoelectric surface, and means for supplying either a negative or a positive potential to said cathode with respect to said grid.
 8. A device comprising a material having an insulating piezoelectric surface which surface has been set at a uniform potential, means for launching an acoustic wave on said surface, means for momentarily flooding said surface with electrons during the presence of said acoustic wave so as to produce a static potential pattern along said surface corresponding to said launched wave, and output means to sense an output acoustic wave corresponding to said static potential pattern along said surface, said output acoustic wave being a reconsitution of the initial acoustic wave and producible by flooding with electrons said surface with static potential pattern.
 9. In a device containing a piezoelectric insulating surface on which a charge pattern has been established, means for flooding said surface with electrons to generate an acoustic surface wave corresponding to the charge pattern, and means for sensing time-varying signals produced by said acoustic surface wave. 